Pope Benedict XVI during Sunday Angelus prayer at his residence of Castelgandolfo, south of Rome, April 11, 2010. Photograph: Osservatore Romano/REUTERS

Prominent atheists Richard Dawkins and Christopher Hitchens are paying lawyers to investigate the possibility of prosecuting the pope for crimes against humanity, their solicitor confirmed today.

The pair argue that Pope Benedict XVI should be arrested when he visits Britain in September and put on trial for his alleged cover-up of sexual abuse in the Catholic church. Last week a letter emerged from 1985 in which the then Cardinal Joseph Ratzinger urged that a paedophilic priest in America not be defrocked for the "good of the universal church".

The Vatican has already suggested the pope is immune from prosecution because he is a head of state. But Dawkins and Hitchens believe that because he is not the head of a state with full United Nations membership, he does not hold immunity and could be arrested when he steps on to British soil.

This is the advice they have been given by their lawyers – solicitor Mark Stephens and human rights barrister Geoffrey Robertson QC.

"I'm convinced we can get over the threshold of immunity," said Stephens. "The Vatican is not recognised as a state in international law. People assume that it has existed for time immemorial but it was a construct of Mussolini, and when the Vatican first applied to become a member of the UN, the US said no. So as a sop they were given the status of permanent observers rather than full members."

But the Holy See insists it is a state like any other. Earlier this month, Giuseppe Dalla Torre, Vatican tribunal chief, said: "The pope is certainly a head of state and he has the same legal status as all heads of state."

Stephens said there are three lines of approach to put the pope in the dock. "One is that we apply for a warrant to the international criminal court. Alternatively, criminal proceedings could be brought here, either a public prosecution brought by the Crown Prosecution Service or a private prosecution. That would require at least one victim to come forward who is either from this jurisdiction or was abused here. The third option is for individuals to lodge civil claims," said Stephens.

He said he had recently been approached by seven wealthy individuals who donated money to the Catholic church and were dismayed their money had not only been used to fund abuse but also buy the silence of victims. These people could potentially sue the pope, Stephens suggested.

Writing in the Washington Post on Friday, Dawkins described Ratzinger as a "leering old villain in a frock … whose first instinct when his priests are caught with their pants down is to cover up the scandal and damn the young victims to silence."

Without admitting that he had consulted lawyers he added: "This former head of the Inquisition should be arrested the moment he dares to set foot outside his tinpot fiefdom of the Vatican, and he should be tried in an appropriate civil – not ecclesiastical – court. That's what should happen. Sadly, we all know our faith-befuddled governments will be too craven to do it."

Pope Benedict will be in Britain from 16-19 September where he will beatify the theologian Cardinal John Henry Newman.